This invention relates in general to aluminum alloys and in particular to a new and useful process for casting aluminum alloys.
The invention relates particularly to a process for casting aluminum alloys, i.e. hypoeutectic aluminum alloys, that contain more aluminum than corresponds to the eutectic with the other alloy constituents in order to achieve better strength values by reducing the secondary dendrite arm spacing upon solidification.
It is known that the technical properties of hypo-eutectic aluminum alloys, particularly tensile strength, yield strength and elongation percent, can be improved by refining the grain of the casting. It is known that the strength properties of aluminum alloys are directly related to the number and fineness of the smallest possible dendrite arm intervals, the secondary dendrite arm spacing. According to Foundary, June 1963, pp. 78-82, the grain fineness of aluminum and aluminum-base alloys is improved by adding a pre-alloy to the aluminum alloy before casting that contains titanium diboride, for example, as heterogeneous seeds (nuclei).
From U.S. Pat. No. 3,259,948, it is known that the grain fineness of castings of cobalt- and nickel-base alloys can be improved by introducing seeds onto the inner surface of the casting mold. These seeds, e.g., cobalt aluminate and cobalt silicate, are applied pursuant to the U.S. patent, to the wax pattern and partly embedded in the inner surface of the mold by then dipping the wax pattern in a slurry of refractory mold material (dipping) and melting away the wax pattern. According to U.S. Pat. No. 3,019,497, seeds, again for the purpose of grain refinement, are mixed with the refractory material for dipping and applied to the wax pattern. According to U.S. Pat. No. 3,158,912, precious metals or reducible metallic oxides are added as seeds to the refractory material for dipping in the same manner as in the other patent mentioned. U.S. Pat. No. 3,157,926 works the same way and mentions nickel (III) oxide, cobalt (II) oxide and (III) oxide and nickel-cobalt hydroxide as the seeds. The seeds used in the above-mentioned U.S. patents are not effective in and are not proposed by the U.S. patents for reducing the secondary dendrite arm spacings and hence for improving the strength properties of hypo-eutectic aluminum alloys. For aluminum-base melts there are still no corresponding seeds that are suitable for embedding in the wall of the mold in order to produce a fine-grained casting. According to Foundry, 1963, the grain fineness of aluminum alloys is improved by adding seeds with the pre-alloy. This is unsatisfactory, however, in terms of reliability and obtaining the smallest possible secondary arm spacings.
German Pat. No. 963,642 teaches influencing the surface of the casting by means of additives to the mold material and alloys the surface with lead released by chemical reaction with the casting material. In order to avoid decarbonization of the skin, according to German Pat. No. 1,271,909, protective reducing materials that have melting points that lie between the casting temperature and the firing temperature of the casting mold are added to the mold material. German Pat. No. 1,265,356 discloses a method whereby the cavity of the mold is treated with a metallic hydride that releases hydrogen. The hydrogen is intended to reduce the oxide skin of the entering casting material, iron, for example, and thus increase flowability. It is known that the aluminum oxide of the cast skin of aluminum is not reducible by hydrogen. Furthermore, the presence of hydrogen during the casting of aluminum alloys is highly undesirable since it can cause the creation of gas bubbles.
The above mentioned German patents teach only that one can have an effect on the surface of the casting with the aid of substances introduced into the mold. The problem of achieving better grain refinement, particularly small secondary dendrite arm spacings in hypo-eutectic aluminum alloys is neither discussed nor solvable by the measure mentioned therein.